Tube comp/limiter under development

[NewYorkDave]

My latest design project... It's still in the "sloppy breadboard" stage, but I have high hopes for it.

It features input and output level controls, a switchable sidechain filter, slope (ratio), threshold, and attack and release controls with a selectable "auto" mode (dual time-constant).

(The panel and meter came from an old Genrad sound and vibration analyzer).

Attack is variable from 0.5mS to 100mS, and release from 47mS to 1Sec.
Here's the envelope of a short tone burst with the attack control set to minimum.

 

[AnalogPackrat]

Go, Dave, go! That is one cool looking front panel, too. Let's see...4 E-I inductive devices--one power and maybe a choke, two audio? That's a pretty quick response time, too. Nice.

A P

 

[NewYorkDave]

Those are all audio transformers... the power supply is off-board :grin:

 

[rafafredd]

Yeah, that looks wonderful. Also, lots of iron on this one!

 

[SSLtech]

Hmmm. bit of overshoot on the GR, but that's really splitting hairs...

:shock: :shock: :shock: NICE WORK, Dave! :shock: :shock: :shock:

I can almost smell the mu varying... -or is it the Knishes? :wink:

:thumb: Keef

 

[AnalogPackrat]

[quote author="NewYorkDave"]Those are all audio transformers... the power supply is off-board :grin:[/quote]

:shock: I guess nobody will be making a stereo one with all Lundahl iron anytime soon, then! Now you've really got me curious about the topology. So what was behind that HP(?) panel in it's prior incarnation?

 

[PRR]

> Hmmm. bit of overshoot on the GR,

So get a few THAT Corp chips. Or build a Fairchild.

This is a 3-bottle limiter. You don't get perfection in 3 bottles.

He's getting 20+dB GR in 0.5mS. You won't hear 0.5mS of tape/ADC clipping. If you have ribbon-shutter film-sound, or trouble with the FCC, you put a clipper after this limiter to whack the overage.

20dB in 0.5mS is fabulous performance for such a simple sidechain built in hollow-state.

It undershoots 1dB but so what?

 

[NewYorkDave]

Yep, it's a hollow-state sidechain. Solid-state could have been used here to good effect, but (for whatever perverse reason) I was going for an all-tube circuit and the extra tube and transformer were the price I was willing to pay. Since the Edcor xfmrs are so inexpensive, I didn't hestitate to use one where it would improve performance, and didn't expend as much effort as I might have otherwise in finding work-arounds to avoid transformers.

The topology can be described loosely as a UA175-like gain control stage followed by an Altec 436-like output stage, with a separate dedicated two-stage amp driving the sidechain. There are four audio transformers in all--input, interstage, output, and sidechain (driving the full-wave rectifier).

Funny thing is, I started this project with the intention of building a simple two-transformer (not counting power), two- or three-tube job along the lines of a 436. But it quickly became apparent to me WHY all those multi-tube, multi-transformer monster limiters were built back in the day: if you want fast limiting, and not just slow AGC, you need a few extra parts...

Look at the people popping up in this thread... It's starting to feel like old times again :grin:

 

[Larrchild]

I found a cool way to adjust a mu limiter's cathodes with no test equipment other than an output meter. 20hz sinewave. Adjust for best symmetry on scope, which coincides with max level. It puts you very close to best THD, noise, and thumplessness.

I also found I could achieve overshoot and undershoot quite easily when out of balance.

Nice Work Dave!

 

[pstamler]

Looks remarkably like a winner, Dave. Good stuff.

Peace,
Paul

 

[Svart]

very cool NYD!

 

[NewYorkDave]

I found a cool way...

That is unusual. As you know, the typical approach is to adjust for best null on a common-mode signal.

I'll have to give your method a try!

 

[Rossi]

:shock: Wow Dave, this thing looks super cool! :shock:

 

[SSLtech]

[quote author="PRR"]It undershoots 1dB but so what?[/quote]
...which is why I was sure to add that it was really splitting hairs!

-I mean, if that's the worst that it does (fastest attack) then it's a VERY attractive proposition indeed!

Keith

 

[tk@halmi]

Coooool stuff!!!! This is very inspiring. Thanks Dave!

I have a sidechain circuit here that I used to play with using a single opamp for gain, a WSM600/600 for phase splitting and 1N34 germanium diodes bridge for rectum-fication. I had a hard time comitting to a tube for vari mu tasks so it is all just sitting in a box.

 

[alk509]

Right f'ing on, Dave!!! :thumb:

Peace,
Al.

 

[ioaudio]

yeah, nice one! finally i´m not the only one who builds stuff on wood...
looking forward to reading some mor details :thumb:

 

[moogah]

Gdammit. Another project I feel compelled to build.

I love compressors.

I love tubes.

Thanks again Dave :thumb:

 

[NewYorkDave]

Here's a block diagram

8kB PDF

 

[analag]

I remember thinking of something close to that. Yours is a little stepped up I think.
http://groupdiy.twin-x.com/albums/userpics/10049/Mu.JPG
http://www.groupdiy.com/index.php?topic=16063&highlight=comp+again

analag